U.S. patent application number 10/506542 was filed with the patent office on 2005-10-13 for device for bending an optical beam, in particular in an optical integrated circuit.
Invention is credited to Bonati, Camilla, Romagnoli, Marco, Tormen, Maurizio.
Application Number | 20050226561 10/506542 |
Document ID | / |
Family ID | 27771819 |
Filed Date | 2005-10-13 |
United States Patent
Application |
20050226561 |
Kind Code |
A1 |
Romagnoli, Marco ; et
al. |
October 13, 2005 |
Device for bending an optical beam, in particular in an optical
integrated circuit
Abstract
A device for varying the direction of an optical beam. A first
waveguide is directed along a first direction, a second waveguide
is directed along a second direction different from the first
direction, and a bending region is interposed between the first and
the second waveguide. The bending region has a photonic crystal
having a regular periodicity and at least a first and a second
crystal axes substantially aligned with the first and second
directions, and a reflecting surface delimiting the photonic
crystal and so positioned and oriented as to reflect an optical
beam coming from the first waveguide toward the second
waveguide
Inventors: |
Romagnoli, Marco; (Milano,
IT) ; Tormen, Maurizio; (Belluno, IT) ;
Bonati, Camilla; (Milano, IT) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER
LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Family ID: |
27771819 |
Appl. No.: |
10/506542 |
Filed: |
May 5, 2005 |
PCT Filed: |
March 6, 2002 |
PCT NO: |
PCT/EP02/02465 |
Current U.S.
Class: |
385/39 ;
385/50 |
Current CPC
Class: |
G02B 6/1225 20130101;
G02B 6/125 20130101; G02B 2006/12104 20130101; B82Y 20/00
20130101 |
Class at
Publication: |
385/039 ;
385/050 |
International
Class: |
G02B 006/26 |
Claims
What is claimed is:
1-9. (canceled)
10. A device for varying the direction of an optical beam,
comprising: a first waveguide directed along a first direction; a
second waveguide directed along a second direction different from
the first direction; and a bending region interposed between the
first and the second waveguide, said bending region comprising: a
photonic crystal having a regular periodicity and having at least a
first and a second crystal axes substantially aligned with said
first and second directions, respectively; and a reflecting surface
delimiting said photonic crystal and so positioned and oriented as
to reflect an optical beam coming from the first waveguide toward
the second waveguide.
11. The device according to claim 10, wherein the photonic crystal
comprises a slab of dielectric material and said reflecting surface
is realized by removing a portion of said slab.
12. The device according to claim 10, wherein said first and second
directions are perpendicular to each other.
13. The device according to claim 12, wherein the photonic crystal
has a periodic array of holes arranged according to a square
geometry.
14. The device according to claim 10, wherein said first and second
directions define an angle of .pi./3.
15. The device according to claim 14, wherein the photonic crystal
has a periodic array of holes arranged according to a triangular
geometry.
16. The device according to claim 10, wherein at least one of said
first and second waveguides is an optical integrated waveguide.
17. The device according to claim 10, wherein at least one of said
first and second waveguides is an optical fibre.
18. The device according to claim 10, wherein the photonic crystal
is made of a bulk material with a first refractive index and has a
periodic array of regions with a second refractive index different
from the first and with predetermined radial dimensions; said
optical beam having a wavelength so related to the difference
between said first and second refractive indices, to the radial
dimensions of said regions and to the period of said array that,
starting from a isotropic distribution of the wave vectors of said
electromagnetic radiation with a first angular range that is twice
the angular extension of the first irreducible Brillouin zone of
said photonic crystal, the group velocity vectors corresponding to
said wave vectors are rearranged during propagation in said
photonic crystal that at least 50% of the group velocity vectors
become directed within a second angular range that is about
one-third of said first angular range and the width at half-maximum
of the distribution of the modules of said group velocity vectors
is lower than about two thirds of said angular range.
Description
[0001] The present invention relates to a device for bending, i.e.
varying the direction of, a guided optical light, in particular for
use in telecommunication optical integrated circuit.
[0002] In optical telecommunication systems, information is
typically coded in short optical pulses by suitable optical
sources, such as light-emitting diodes (LEDs) or semiconductor
lasers, which pulses are transmitted along an optical-fibre network
and received by photodetectors. Many different signals can be
transmitted using a single wavelength of light by interweaving the
pulses from different sources, a technique known as time-division
multiplexing (TDM).
[0003] A simple way of increasing the amount of data that can be
transmitted by a single optical fibre is to make the incoming
electronic bits as short as possible. Current optical systems have
achieved data rates up to 40 gigabits per second.
[0004] Recently, transmission capacity has been increased by dense
wavelength division multiplexing (DWDM), which requires a very
stable emitting laser and, at the receiver, very narrow linewidth
filters and optical switches for separating individual wavelength
channels and routing them to the appropriate destinations. Due to
the large number of individual components in a DWDM system,
integrated optical circuits have been developed. Integrated optical
circuits may be either monolithic or hybrid and comprise active and
passive components, typically realized on a semiconductor or
dielectric substrate, used for coupling between optoelectronic
devices and providing signal processing functions.
[0005] As described by R. L. Espinola et al. in "A study of
high-index-contrast 90.degree. waveguide bend structures", OPTICS
EXPRESS, Vol. 8, No. 9, pp. 517-528, 23 Apr. 2001, low-cost,
highly-functional optical integrated circuits require a material
system with relatively high refractive index contrast in order to
increase the packing density of the optical elements, and also the
use of sharp, e.g. 90.degree., bends. However, low-loss sharp bends
cannot be easily achieved with standard waveguide technology
because waveguide loss increases exponentially with the inverse
bend radius.
[0006] New approaches to achieving sharp bends have therefore been
considered.
[0007] For example, it has been proposed to use waveguide corner
mirrors, which exploit strong modal confinement and total internal
reflection (TIR) at the corner, as described by W. Yang and A.
Gopinath in "Design of planar optical waveguide corners with
turning mirrors", Proceedings of Integrated Optics, Technical
Digest Series, Vol. 6 (optical Society of America, Washington,
D.C., 1996), pp. 58-63. With suitable mirror placement and angle,
these structures can reflect the incident light with low-loss in
the bend.
[0008] Another widely-studied approach to low-loss bends is that
using high-index-contrast waveguides, whose strong light
confinement properties allow performing complex waveguide
interconnections within a small area. C. Manolatou et al, in
"High-Density Integrated Optics", Journal of Lightwave Technology,
Vol. 17, No. 9, September 1999, pp. 1682-1692, show that by
modifying the waveguide intersection regions into resonant
structures with simmetry, right angle ends, T-junctions, and
crossing with high transmission characteristics are also possible
using conventional single-mode high index-contrast waveguides.
[0009] The Applicant observes that the very small dimensions of
high-index-contrast waveguides cause difficulties in coupling light
at their input and relatively high scattering losses.
[0010] Recently, the use of photonic crystal waveguides has been
proposed for making high transmission 90.degree.-bends, as
described by A. Mekis et al. in "High transmission through sharp
bends in photonic crystal waveguides", Phys. Rev. Lett. 77,
3787-3790 (1996). In this case, photonic band-gap (PBG) materials
are modified by inserting a line of defects that can support a
localized mode having a frequency located within the photonic
bandgap, as described by R. D. Meade et al. in "novel application
of photonic band gap material: Low-loss bends and high Q cavities",
J. Appl. Phys. 75, 4753-4755 (1994). The defect line thus supports
a local state and acts as a waveguide. An efficiency near to 100%
has been observed at certain frequencies near the valence band edge
for .lambda..about.1.55 .mu.m for a 60.degree. photonic-crystal
waveguide bend in E Chow et al., "Quantitative analysis of bending
efficiency in photonic-crystal waveguide bends at .lambda.=1.55
.mu.m wavelengths", Optics Letters, Vol. 26, No. 5, Mar. 1, 2001,
pp. 286-288.
[0011] U.S. Pat. No. 5,526,449 discloses an optical circuit and a
method for substantially eliminating radiation losses associated
with optical integrated circuits and, in particular, bends in
optical waveguides. The circuit and waveguide are fabricated on a
substrate having a periodic dielectric structure. The periodic
dielectric structure exhibits a range of frequencies of
electromagnetic radiation that cannot propagate into the structure,
i.e. a photonic band gap. Radiation at a frequency within the
frequency band gap of the structure is confined within the circuit
and waveguide by the periodic dielectric structure surrounding the
circuit and waveguide.
[0012] U.S. Pat. No. 6,134,369 describes a compact optical
waveguide employing a photonic band gap element as a reflector to
enable a light beam to be reflected at angles greater than the
critical angle. The photonic band gap element is a two-dimensional
array of columnar holes formed in the substrate, the holes being
filled with air or another material having a different dielectric
constant than the substrate. The optical waveguide forms a right
angle bend and first and second photonic band gap elements are
formed on the inside and outside of the bend to deflect light which
is incident on the waveguide at an angle greater than a critical
angle defined by the materials that constitute the optical
waveguide.
[0013] Besides development of photonic crystals devices having
defects, behaviour of light in photonic crystals having regular
periodicity, herein below referred to as "regular photonic
crystals" for simplicity, has been investigated. For the purposes
of the present invention, with "photonic crystals having regular
periodicity" it is intended a photonic crystal wherein the
characteristics of its periodic array do not vary at least in a
region thereof of intended light propagation.
[0014] The article of P. Etchegoin and R. T. Phillips, "Photon
focusing, internal diffraction, and surface states in periodic
dielectric structures", Physical Review B, Volume 53, Number 19, 15
May 1996-1, takes advantage of some analogies between electrons in
semiconductors and electromagnetic waves in periodic dielectric
structures for providing a method for calculation the band
structure of a 2-D periodic dielectric structure. Moreover, this
article deals with the phenomenon of photon focusing emitted by a
source point in these structure, in analogy with the phenomenon of
acoustic phonon focusing, showing what shape shall have the
k.sub.x-k.sub.y diagram of the wave vector k to have focusing of
light along predetermined directions.
[0015] The Applicant observes that the phenomenon of photon
focusing, which has been studied only at theoretical level, would
find only limited applications in integrated optics.
[0016] The article of Marko Lon{haeck over (c)}ar, Jelena Vu{haeck
over (c)}kovi{haeck over (c)} and Axel Scherer, "Three-dimensional
analysis of dispersion properties of planar photonic crystals",
Proceedings of PECS III conference (June 2001), St. Andrew's,
Scotland shows that a planar (i.e. 2-D) photonic crystal may have,
under certain conditions, a self-collimation effect in the second
energy band (i.e. the energy band over the fundamental band). As
disclosed in the article, these conditions determine a negative
group velocity.
[0017] The Applicant observes that, for the time being, no
practical applications have been shown of a beam of light (although
collimated) having a negative group velocity.
[0018] The Applicant has found that high-efficiency sharp bending
in an integrated optical circuit can be obtained by using an
opportunely designed photonic crystal associated to a reflecting
surface. In fact, the Applicant has found that a photonic crystal
can be made suitable to guide a beam of light in a collimated way
in a portion thereof of regular periodicity, if an opportune
relation exists between predetermined crystal properties and the
considered wavelength, and has moreover found that such a crystal
can be provided with a reflecting surface so as to realize a device
for sharp bending with negligible losses an optical beam between
two waveguides.
[0019] Therefore, the present invention relates to a device for
varying the direction of an optical beam, comprising a first
waveguide directed along a first direction, a second waveguide
directed along a second direction different from the first
direction, and a bending region interposed between the first and
the second waveguide, wherein the bending region comprises a
photonic crystal having a regular periodicity and having at least a
first and a second crystal axes substantially aligned with said
first and second directions, and a reflecting surface delimiting
said photonic crystal and so positioned and oriented as to reflect
an optical beam coming from the first waveguide towards the second
waveguide.
[0020] The photonic crystal preferably comprises a slab of
dielectric material and the reflecting surface is preferably
realized by removing a portion of said slab.
[0021] The first and second directions may be perpendicular to each
other. This situation occurs, for example, when the photonic
crystal has a periodic array of holes arranged according to a
square geometry.
[0022] Alternatively, the first and second directions may define
and angle of .pi./3, for example when the photonic crystal has a
periodic array of holes arranged according to a triangular
geometry.
[0023] At least one of the first and second waveguide may be an
optical integrated waveguide, or alternatively an optical
fibre.
[0024] Preferably, the photonic crystal is made of a bulk material
with a first refractive index and has a periodic array of regions
with a second refractive index different from the first and with
predetermined radial dimensions; the optical beam having a
wavelength so related to the difference between said first and
second refractive indices, to the radial dimensions of said regions
and to the period of said array that, starting from a isotropic
distribution of the wave vectors of said electromagnetic radiation
within a first angular range that is twice the angular extension of
the first irreducible Brillouin zone of said photonic crystal, the
group velocity vectors corresponding to said wave vectors are so
rearranged during propagation in said photonic crystal that at
least 50% of the group velocity vectors become directed within a
second angular range that is about one-third of said first angular
range and the width at half-maximum of the distribution of the
modules of said group velocity vectors is lower than about
two-third of said second angular range.
[0025] Further details may be obtained from the following
description, which refers to the accompanying drawings listed
below:
[0026] FIG. 1 is an illustrative representation of a 2-D photonic
crystal having a regular periodicity;
[0027] FIG. 2a schematically shows a device for propagating a light
beam through a photonic crystal according to the present invention,
and 2b show a possible arrangement (triangular in this case) of the
holes in a photonic crystal;
[0028] FIG. 3 is an illustrative representation of an energy band
diagram for a photonic crystal having a triangular array of
holes;
[0029] FIGS. 4a and 4b show diagrams of wave-vectors k and group
velocities v.sub.g suitable for observing waveguiding in a photonic
crystal;
[0030] FIG. 5 is an illustrative representation of the distribution
of group velocities v.sub.g in the direction of a crystal axis;
[0031] FIG. 6 is an illustrative perspective view of a device for
waveguiding electromagnetic radiation according to the present
invention;
[0032] FIGS. 7a and 7b show possible X-crossing devices made in
accordance to the present invention;
[0033] FIG. 8 shows a device for producing a sharp bend of a light
beam, according to the present invention; and
[0034] FIGS. 9a-9d, 10a-10d, 11, 12 and 13 relate to results of
numeric simulations.
[0035] With reference to FIGS. 1, 2a and 2b, it is indicated with 1
a photonic crystal comprising a bulk body 2 made of a dielectric
material, wherein a periodic array of holes 2 has been realized.
Although a bi-dimensional (2-D) photonic crystal is described, the
teaching of the present, invention may be extended to
three-dimensional (3-D) photonic crystals as well, i.e. crystals
having a 3-D distribution of regions of different refractive index
than the bulk, such as spheres.
[0036] The bulk dielectric material has a refractive index n
preferably greater than 1.5. Possible materiales are SiO.sub.2,
SiO.sub.xN.sub.y, GaAs, Si; this list is only illustrative and not
exhaustive.
[0037] Preferably, holes 2 contain air, are cylindrical and
parallel to each other and have a same radius .rho.. As shown in
FIG. 2b, holes 2 may be arranged to form a triangular array having
a period a. Although an arrangement according to an equilateral
triangle is illustrated, an arrangement according to an isosceles
triangle may be used, as well. Alternatively, holes 2 may be
arranged according to a square or a rectangular array. In general,
the periodicity of the array is the periodicity of the "cells",
each "cell" including three holes for triangular arrays and four
holes in square or rectangular arrays.
[0038] It can be shown that the preferred transmission directions
are those corresponding with the crystal axes. In case of
triangular geometry, the crystal axes are those connecting the
vertices of the triangles, while in case of square or rectangular
geometries the crystal axes correspond to the diagonals of the
square or triangle, i.e. the axes connecting opposite vertex. In
case of triangular array geometry the three crystal axes are
angularly spaced at 60.degree. (.pi./3), while in case of square
array geometry the two crystal axes are angularly spaced at
90.degree. (.pi./2).
[0039] Although the description that follows is referred to a
photonic crystal having holes 2 filled with air and of circular
cross-section, the teaching of the present inventions extends to
photonic crystals wherein the holes contain a predetermined
substance different from air or are substituted by rods of a
predetermined dielectric material (of different refractive index
than the bulk dielectric material), and wherein the holes have
cross-sections other than circular, for example square.
[0040] In accordance with the above, photonic crystal 1 has a
structure that does not vary along an axis z parallel to the axes
of holes 2, and that varies periodically in a plane x-y
perpendicular to axis z.
[0041] Method of manufacturing photonic crystal 1 will not be
described in that methods known in the art can be used. Methods for
manufacturing photonic crystals and devices comprising photonic
crystals are described, for example, in U.S. Pat. No. 5,526,449,
U.S. Pat. No. 6,064,511 and in the article of M. Lon{haeck over
(c)}ar, T Doll, J. Vu{haeck over (c)}kovi{haeck over (c)} and A.
Scherer, "Design and Fabrication of Silicon Photonic Crystal
Optical Waveguides", Journal of Lightwave Technology, Vol. 18, No.
10, October 2000.
[0042] The parameters that determine the guiding characteristics of
the photonic crystal 1 are the relative spacing R, defined as the
ratio between the period a of the array of holes 2 and the radius
.rho. of holes 2, the refractive index n of the dielectric material
(or, in general, the difference of refractive index between the
bulk dielectric material and the substance or material filling the
holes), and a further parameter, known as "normalized frequency"
and indicated with .omega..sub.n, proportional to the ratio between
the period a and the wavelength .lambda. of the electromagnetic
radiation to be guided.
[0043] The condition of "photon guiding", or "waveguiding", is
achieved if the photonic crystal 1 is so designed, and the
electromagnetic radiation propagating therein has such a
wavelength, that a substantial fraction of the energy of the
electromagnetic radiation is made to flow in a direction coinciding
with one of the crystal axes. The "photon guiding" condition will
herein below defined by reference to a well-known vector parameter
of the electromagnetic radiation, the "group velocity", whose
direction corresponds to that of energy propagation. In practice,
starting from an isotropic distribution of group velocities,
"photon guiding" occurs when said distribution is so modified
that:
[0044] a predetermined percentage of the group velocities is
oriented within a predetermined sub-range of the original angular
range;
[0045] the angular distribution of the module of the group
velocities shows a peak along the considered propagation axis, and
has a width at half-maximum lower than a predetermined value.
[0046] In other words, the photon crystal shall be so designed that
the group velocies converge towards the desired propagation
direction and that most of the energy of the propagating beam is
carried by the group velocities so directed.
[0047] For the purposes of the present invention, a condition of
"photon guiding" of an electromagnetic radiation in a predetermined
direction within a photonic crystal is a condition wherein,
starting from a isotropic distribution of the wave vectors of said
electromagnetic radiation within a first angular range that is
twice the angular extension of the first (irreducible) Brillouin
zone of the photonic crystal, the group velocity vectors
corresponding to said wave vectors are rearranged as concerns
direction and module so that at least 50% of the group velocity
vectors become directed within a second angular range that is,
about one-third of the first angular range and the width at
half-maximum of the distribution of the modules of said group
velocity vectors is lower than about two-thirds of the second
angular range.
[0048] In a preferred condition, the width at half-maximum of the
distribution of the modules of said vectors is lower than about
one-half of the second angular range.
[0049] In the description that follows, the angular distribution of
the group velocity will be indicated with as r(.theta.).
[0050] In the previous definition of "photon guiding", as in the
following of the present description, the width at half-maximum of
a distribution r(.theta.) will be considered as equivalent to the
variance of the distribution, wherein the variance .sigma. is so
defined: 1 = i ( i - ) 2 r ( i ) i r ( i )
[0051] where .theta..sub.i indicates the angle with respect to the
considered direction, and .mu. is the average of said distribution
about said direction and is equal to 0 for a triangular array and
to .pi./4 for a square array.
[0052] Preferred conditions for photon guiding may be expressed in
terms of variance .sigma.. In case of a triangular geometry, a
preferred condition of "photon guiding" along a crystal axis occurs
when, starting from a isotropic distribution of the wave vectors
within an angular range of 60.degree. (.pi./3) about said axis
(which is twice the angular extension of the first irreducible
Brillouin zone), the group velocity vectors corresponding to said
wave vectors are rearranged, as concerns module and direction, so
that at least 50% of the group velocity vectors become directed
within an angular range of 20.degree. (.pi./9) about said axis
(i.e. within an angular range that is one-third of the original
one) and the variance a of the module distribution about said axis
is lower than about 0.15 rad (i.e. .sigma..sup.2<.about.0.022
rad.sup.2). A better guiding effect may be observed for .sigma.
lower than about 0.13 rad (i.e. .sigma..sup.2<.about.0.017
rad.sup.2) and a still better guiding effect for .sigma. lower than
about 0.11 rad (i.e. .sigma..sup.2<.about.0.012 rad.sup.2).
Choosing a more or a less restrictive condition (in terms of
.sigma.) may depend on the particular requirements and conditions,
for example on the distance to be covered through the photonic
crystal.
[0053] In case of a square geometry, a preferred condition of
"photon guiding" along a crystal axis occurs when, starting from a
isotropic distribution of the wave vectors within an angular range
of 90.degree. (.pi./2) about said axis (which is twice the angular
extension of the first irreducible Brillouin zone), the group
velocity vectors corresponding to said wave vectors are rearranged,
as concern module and direction, so that at least 50% of the group
velocity become directed within an angular range of 30.degree.
(.pi./6) about said axis (i.e. within an angular range that is
one-third of the original one) and the variance .sigma. of the
module distribution about said axis is lower than about 0.22 rad
(i.e. .sigma..sup.2<.about.0.049 rad.sup.2). A better guiding
effect may be observed for .sigma. lower than about 0.17 rad (i.e.
.sigma..sup.2<.about.0.029 rad.sup.2) and a still better guiding
effect for .sigma. lower than about 0.12 rad (i.e.
.sigma..sup.2<.about.0.014 rad.sup.2).
[0054] Propagation under "photon guiding" further requires that the
electromagnetic radiation has a wavelength in the fundamental
photonic band. It can be shown that the above condition of
"photonic guiding" is achieved where the surface of the fundamental
energy band (in the .omega., k.sub.x, k.sub.y space) shows an
inflection.
[0055] Therefore, differently from waveguiding into conventional
photonic crystals, wherein light at a wavelength within the
photonic band-gap is conveyed through a localized and unchangeable
waveguide typically defined by a line of defects in the photonic
crystal, waveguiding into a photonic crystal according to the
present invention can occur at a wavelength within the photonic
fundamental band and in any region of the photonic crystal of
regular periodicity. In practice, a "virtual waveguide" is created
when a beam of light is made to propagate into a photonic crystal
according to the present invention, since the particular
propagation conditions "observed" by the beam of light force it to
collimate.
[0056] It can therefore be appreciated that the photonic crystal of
the present invention does not require any physical waveguide
structure such as the linear defect regions of conventional
photonic crystals. Dimension and position of a collimated guided
beam in the photonic crystal of the present invention are
determined by, and correspond to, dimension and position of the
input light beam, while the possible propagation directions are
those correspond to the crystal axes.
[0057] Therefore, if photonic crystal 1 is optically coupled to an
optical source suitable to emit electromagnetic radiation at a
predetermined wavelength, and if this wavelength is so related to
the physical characteristics of the photonic crystal 1 to satisfy
the conditions for photon guiding, a device suitable to guide a
beam of light at this wavelength can be realized. FIG. 2a shows
schematically an optical source 4, for example a known type of
laser, optically coupled to photonic crystal 1 by means of a first
optical waveguide 5 of a known type, for example an optical fibre
or an integrated optical waveguide. A second optical waveguide 7 of
a known type is coupled to the photonic crystal 1 opposite the
first waveguide 5. Also schematically represented is a beam of
light 6 generated by the optical source 4 and fed to the photonic
crystal 1 via the first waveguide 5, which is then propagated
substantially collimated into the photonic crystal 1 and exits the
photonic crystal 1 via the second waveguide 7.
[0058] A further important characteristic of the photonic crystal
of the present invention is that a plurality of light beams,
possibly of different dimensions and directions, can be transmitted
simultaneously in the crystal. In particular, it is possible to
transmit two or more light beam in the crystal that cross each
other so as to define a "X-crossing" structure. The Applicant has
verified that in such a crossing there is no interaction (i.e., no
cross-talk) between the electromagnetic fields of the two beams.
Possible X-crossing will be described in the following.
[0059] A same photonic crystal according to the present inventions
may be therefore applied in different ways, for example for guiding
a plurality of parallel light beams evenly or differently spaced
from each other and having the same or different dimensions, or for
guiding a plurality of light beam propagating along different
directions (coinciding with crystal axes) and having the same or
different directions, which may cross each other in a same or in
different regions of the crystal (for example: a beam propagating
in a first direction may cross, one after the other, two further
beams propagating in a second direction; three beams propagating in
three different directions may cross in a same region).
[0060] When a plurality of optical beams are propagated, these
beams may have a same wavelength or different wavelengths, provided
that these wavelengths are suitable for photon guiding in the
considered photonic crystal according to the teaching of the
present invention.
[0061] The Applicant has verified that preferred ranges of the
relevant parameters n, R and .omega..sub.n for achieving the said
condition of "photon guiding" are, for triangular and square array
geometries, the following:
[0062] for triangular array geometry:
[0063] relative spacing R: between 0.15 and 0.5, more preferably
between about 0.20 and 0.45, still more preferably between about
0.25 and 0.4;
[0064] refractive index n: between 1.5 and 4.5, more preferably
between 2.5 and 3.5;
[0065] normalized frequency .omega..sub.n: between about 0.17 and
0.42, more preferably between about 0.25 and 0.40.
[0066] for square array geometry:
[0067] relative spacing R: between 0.15 and 0.5, more preferably
between about 0.20 and 0.45, still more preferably between about
0.25 and 0.4;
[0068] refractive index n: between 1.5 and 4.5, more preferably
between 2.5 and 3.5;
[0069] normalized frequency .omega..sub.n: between about 0.17 and
0.42, more preferably between about 0.25 and 0.40
[0070] As described in the following, there is a relation among
these three parameters, so that when two of the three parameters
are chosen, the third is fixed.
[0071] To find the optimum value ranges of the relevant parameters
R, n and .omega..sub.n, the following method can be followed.
[0072] The method comprises analysing the group velocity
distribution resulting from the possible combinations of values of
the parameters R, n and .omega..sub.n within predetermined ranges
and determining the set of combinations that provide a group
velocity distribution suitable to have photon guiding according to
the above teaching.
[0073] To this aim, a theoretical model the crystal, such as the
model proposed in the above-cited article of P. Etchegoin and R. T.
Phillips, may be advantageously applied. This model can be
initially used for determining the band diagram of the crystal when
a couple of values of n and R has been established and can be then
used to determine the distribution of the group velocities when
predetermined values of R, n and .omega..sub.n are considered.
[0074] If a 2-D dielectric structure as in FIG. 1 is considered, in
particular having an extension along the x and y axes much greater
than along the z-axis, an electromagnetic band structure as in the
illustrative representation of FIG. 3 is obtained, where .GAMMA., M
and K are the well-known parameters identifying the first
Irreducible Brillouin zone, and the continuous and dashed lines
correspond to the TE and TM modes, respectively. As concerns the TE
modes, a fundamental photonic band (or first band), here indicated
with BAND I, and a photonic band-gap PBG, separating the
fundamental band BAND I from a second photonic band BAND II, may be
identified.
[0075] As known from theory of electromagnetic propagation in
anisotropic media, energy does not flow in the same direction of k.
When energy is represented by dispersion relation .omega.(k), it is
easily demonstrated that energy flows in the direction of the group
velocity v.sub.g=.gradient..sub.k.multidot..omega.(k). In fact, it
can be shown formally that v.sub.g, obtained as a gradient of
.omega.(k), coincides with the direction of the Poynting vector
S.varies.E.times.H for electromagnetic waves. Therefore, the
electromagnetic band structure provides a relation
.omega.=.omega.(k), from which the group velocity v.sub.g can be
obtained.
[0076] For a predetermined energy it is possible to define two
surfaces: the constant energy surface, represented in the
k.sub.x-k.sub.y plane, obtained directly by the intersection of the
3-D band surface (in the space .omega., k.sub.x, k.sub.y) with a
plane with constant abscissa equal to given energy value; and the
wave surface, formed by the locus of all possible group velocities
and represented in the v.sub.gx-v.sub.gy plane. The wave surface
represents the wave front that would emerge from a point source. It
can be shown that, for each point of the constant energy surface,
the corresponding group velocity is directed perpendicular to said
surface. Said article P. Etchegoin and R. T. Phillips illustrates
possible shapes for these surfaces for photon focusing.
[0077] In order to have a collimated guiding of light within the
crystal, these surfaces shall have particular shapes, in particular
the constant energy surface (in the k.sub.x-k.sub.y plane) shall
have straight portions with the broadest possible extension, and
the group velocities shall be directed outwards with respect to the
centre of the diagram. The wider and the flatter are said portions,
the larger will be the range of group velocities having the same
direction (i.e. the more collimated will be the guided light). In a
triangular array geometry, to have a substantially collimated
transmission, the constant energy surface shall have a
substantially hexagonal shape, with straight portions defining the
edges of the hexagon. Differently, in a square array geometry, the
constant energy surface shall have a substantially square shape,
and the group velocities will be highly concentrated in the
directions perpendicular to the four edges, i.e. in four different
directions equally angularly spaced (of .pi./2).
[0078] FIG. 4a shows, illustratively, a constant energy surface
(k.sub.x-k.sub.y plane) of a type suitable for having collimated
guiding in case of a triangular array geometry and for the TE mode
of a monomode electromagnetic radiation, and FIG. 4b shows the
corresponding wave surface (v.sub.gx-v.sub.gy plane).
[0079] In the simulation, a isotropic distribution of group
velocities about a predetermined crystal axis in an angular range
of .pi./3 for a triangular array, and of .pi./2 for a square array,
has to be considered as a starting condition, and the parameters R,
n and .omega..sub.n may be varied for example as follows.
[0080] The parameter R may be varied in a range that is chosen by
taking into account the current technological difficulties in
realizing large aspect ratio holes arrays in photonic crystals. A
suitable range is for example between 0.15 and 0.45.
[0081] The parameter n may be is varied in a range of values
including those of the currently most suitable dielectric materials
for the manufacturing of photonic crystals, such as SiO.sub.2,
SiO.sub.xN.sub.y and GaAs: for example the values 1.5, 2, 2.5, 3.25
and 3.5 may be considered.
[0082] The third parameter .omega..sub.n may be varied of a
predetermined amount (for example 1.multidot.10.sup.-4) in a
predetermined range, for example starting from the maximum value
.omega..sub.n,max of the first band, until a predetermined
condition is satisfied, as explained below.
[0083] The simulation model previously mentioned may be
advantageously applied in this stage to each couple of values of R
and n within the specified ranges, determining each time the value
of .omega..sub.n that optimizes the distribution of the group
velocity r(.theta.).
[0084] In particular, for each couple of values of R and n, the
parameter .omega..sub.n is varied of the predetermined amount until
the group velocity distribution r(.theta.) is optimised in terms of
average .mu., variance .sigma., height .DELTA.h, kurtosis K and
percentage of representative points that are within a predetermined
angular range .DELTA..theta. about the considered direction. This
angular range, as previously stated, is chosen equal to .pi./9 for
a triangular array geometry and .pi./6 for a square array geometry.
In particular, the optimum value of .omega..sub.n is the one that
minimizes .sigma. and K and maximizes .DELTA.h and the percentage
of points within the angular range .DELTA..theta.. FIG. 5 is an
illustrative representation of a group velocity distribution, with
an indication of the said parameters, which are defined as follows
(definition of a is reported again for sake of clarity): 2 h = max
( r ( ) < ) - min ( r ( ) < ) min ( r ( ) < ) ; = i ( i -
) 2 r ( i ) i r ( i )
[0085] where .mu.=0 for a triangular array and .mu.=.pi./4 for a
square array; and 3 K = i ( i - ) 4 r ( i ) i r ( i ) .
[0086] When .omega..sub.n is equal to its optimum value, the curve
of the group velocity distribution is relatively sharp and has the
maximum height .DELTA.h. If .omega..sub.n is varied in one
direction or the other with respect to its optimum value, the
distribution undergoes an abrupt decrease of .DELTA.h, and an
increase of the kurtosis K and of the variance .sigma.. In
particular, variations of the parameter .omega..sub.n with respect
to said optimum condition determine either a broadening of the
distribution or the appearance of a ring in the curve, and it can
be demonstrated that both conditions are undesirable.
[0087] When the optimum value of .omega..sub.n is found for a
predetermined couple of values of n and R, it is checked if the
values of the variance ar and of the percentage of points within
the specifies angular range for the considered tern of values of n,
R and .omega..sub.n satisfies the above-mentioned conditions for
photon guiding.
[0088] All the terms of value of R, n and .omega..sub.n so detected
identify a crystal structure, a bulk material and a wavelength
suitable to have photon guiding.
[0089] FIG. 6 illustrates an integrated optical device 200
comprising a substrate 207 and a plurality of layers superimposed
on a z direction, in particular a decoupling layer 202, a first
cladding layer 203, a core layer 204 and a second cladding layer
205. The refraction index of the core layer 204 is higher than that
of the first and second cladding layers 203, 205. The refractive
index of the decoupling layer 202 is lower than that of the first
cladding layer 203.
[0090] Moreover, device 200 has a periodic array of holes 2 that
cross layers 205, 204, 203 and 202 along the z direction down to
the substrate 207, thus defining a photonic crystal 201. Photonic
crystal 201 is designed according to the present invention. Besides
providing a photon guiding effect in a x-y plane, device 200 also
confines light in the z direction inside the core layer 204, due to
the difference of refraction index of the core layer 204 with
respect to the first and second cladding layers 203, 205.
[0091] Photonic crystal 201 may be interfaced with different types
of conventional waveguides for the input and the output of light,
such as ridge waveguides (with shallow edge or deep edge, the
latter being also known as "mesa"), rib waveguides, photonic
crystal waveguides (with linear defect regions). The input and
output of light may be also performed by means optical fibers, in
particular by facing the core of the fiber to the core layer 204.
In the illustrative example of FIG. 6, device 200 includes a mesa
waveguide 210 integrally connected to the photonic crystal 201.
Mesa waveguide 210 extends along a direction corresponding to one
of the crystal axes of the photonic crystal 201 and has a
succession of layers corresponding to those of photonic crystal 201
so that, in the manufacturing process, it may be grown together
with photonic crystal 201.
[0092] As an example, a device suitable to operate at 1550 nm has a
substrate 207 made of GaAs having a refractive index (measured at
25.degree.) of 3.374, a core layer 204 made of
Al.sub.0.37Ga.sub.0.63As having a refractive index (measured at
25.degree. ) of 3.25, first and second cladding layers 203, 205
having a refractive index of 0.01 lower than that of the core layer
204 (by using a different percentage of Al), and a decoupling layer
202 that, in turn, has a refractive index of 0.01 lower than that
of the cladding layers 203, 205 (by using a different percentage of
Al). The characteristic parameters of the considered photonic
crystal are n=3.25, R=0.42 and .omega..sub.n=0.26. The radius .rho.
of holes 2 is 93 nm and the period a of the array is 311 nm. Holes
2 are arranged according to a triangular array and are filled of
air.
[0093] Such a device provides a k.sub.x-k.sub.y and
v.sub.gx-v.sub.gy diagrams as those represented in FIGS. 4a and
4b.
[0094] Device 200 is only an illustrative example and, in general,
a photonic crystal according to the present invention may be used
in any type of planar waveguiding structure able to confine light
in the z direction.
[0095] FIG. 7a and 7b are illustrative representation of two
different X-crossing devices 300, 400 that make use of a photonic
crystal according to the present invention, with a triangular and
square arrangement of holes 2, respectively. In particular, device
300 comprises a hexagonal photonic crystal structure 301 having a
periodic array of holes 2 with equilateral triangular array
geometry, and three couples of opposite waveguides 302, 303, 304
directed along the crystal axes, here indicated with x.sub.1,
x.sub.2 and x.sub.3. Each couple of waveguides 302, 303, 304
includes an input waveguide 302', 303', 304' and an output
waveguide 302", 303", 304". Waveguides 302, 303 and 304 may be, for
example, conventional waveguides in integrated optics, or be
defined by linear defects regions of a photonic crystal.
Alternatively, in place of one or more of waveguides 302, 303 and
304, there may be a corresponding optical fiber (not shown) facing
the edge of the photonic crystal.
[0096] In use, three different beams of light A, B, C, carrying
respective signals and having a same wavelength or close different
wavelengths suitable for photon guiding, are fed to the photonic
crystal 301 via the input waveguides 302', 303', 304' and propagate
collimated into the photonic crystal 301. In the centre of the
photonic crystal 301, the three beams A, B, C cross each other. It
has been verified that this crossing occurs without mutual
interaction among the beams, i.e. without cross-talk. After
crossing, the beams continue their propagation to reach the
respective output waveguides 302", 303", 304" so that they can exit
the device.
[0097] A device with an isosceles triangle array geometry may
alternatively be used, and the directions of the input and output
waveguides shall be varied accordingly, in order to coincide with
the directions of the crystal axes.
[0098] It can be appreciated that a couple of waveguide may be
omitted from device 300, so as to have a device that is simpler but
is suitable for crossing only two beams of light.
[0099] Device 400 comprises a square photonic crystal structure 401
having a periodic array of holes 2 arranged according to a square
geometry, and two couples of opposite waveguides 402, 403 directed
along the crystal axes, here indicated with x and y. Each couple of
waveguides 402, 403 includes an input waveguide 402', 403' and an
output waveguide 402", 403". Waveguides 402, 403 may be, for
example, conventional waveguides in integrated optics, or be
defined by linear defects regions of a photonic crystal.
Alternatively, in place of one or both of waveguides 402, 403,
there may be a corresponding optical fiber (not shown) facing the
edge of the photonic crystal.
[0100] In use, two different signals A and B are fed to the
photonic crystal 401 via the input waveguides 402', 403', propagate
collimated into the photonic crystal 401, cross each other in the
centre thereof without mutual interaction, and continue their
propagation by entering the output waveguides 302", 303", 304".
Therefore, two different beams of light can cross each other
without crosstalk along two directions at 90.degree. to each
other.
[0101] A device comprising a rectangular structure and/or a
rectangular geometry of the array of holes 2 may be used as
well.
[0102] FIG. 8 shows a device 500 for sharply bending a beam of
light. Device 500 comprises a slab 501 of dielectric material of
refractive index n, that defines, in a portion thereof, a photonic
crystal 502 according to the present invention. In the particular
example here illustrated, the photonic crystal 502 has a periodic
array of holes 2 with square geometry and has therefore two
perpendicular crystal axes x, y. One portion of slab 501 has been
removed by known techniques so as to define an air gap 505, i.e. a
region of space with refractive index n=1. The interface between
the photonic crystal 502 and the air gap 505 defines a reflecting
surface 506. Surface 506 is so oriented that the normal to surface
506 bisects the two crystal axes x, y. The region occupied by the
photonic crystal 502 and delimited by reflecting surface 506
defines a bending region of the device.
[0103] Device 500 further comprises a first (or input) and a second
(or output) waveguide 507, 508 of a known type, made of a material
having a second refractive index n.sub.2 different from n.sub.1.
First and second waveguides 507, 508 are directed along the crystal
axes x and y, respectively, and end at the borders of the photonic
crystal 502.
[0104] In use, one of the two waveguides, for example the first
waveguide 507, conveys a beam of light A into the photonic crystal
502. The photonic crystal 502, if the wavelength of the beam A is
so related to the refractive index n.sub.1 and to the period and
radius of the holes 2 as previously described (i.e. so as to
satisfy the "photon guiding" condition), guides the beam A along
axis y to the reflecting surface 506 and, after reflection, along
axis x to the second waveguide 508, where it is newly conveyed in a
conventional way.
[0105] It can be appreciated that the reflecting surface may be
defined in different ways, for example by providing an interface
different from air.
[0106] The array of holes may also have a different geometry, such
as rectangular or triangular. In case of triangular array geometry,
reflection may occur at different angles, provided the normal to
the reflecting surface bisects the two considered propagation axes;
for example, when holes are arranged according to an equilateral
triangle, reflection may occur at 60.degree. (.pi./3) or
120.degree. (.pi./3). Input and output waveguides shall be oriented
accordingly.
[0107] Waveguides for the input and the output of light may be any
type of known integrated optical waveguides, such as ridge
waveguides (with shallow edge or deep edge, the latter being also
known as "mesa"), rib waveguides, or photonic crystal waveguides
(with linear defect regions).
[0108] As an alternative, one or both of the input and output
waveguide may be an optical fibre facing laterally the slab
comprising the photonic crystal.
EXAMPLE 1
[0109] In a first set of simulations, it is shown how the
characteristics parameters R, n and .omega..sub.n can be selected
for achieving photon guiding. The simulation model previously
described has been used for determining the group velocity
distributions for different combination of values of said
parameters. A photonic crystal having a (equilateral) triangular
array of air holes and an initial isotropic distribution of the
group velocities has been considered. The analysis is limited to
the first dispersion band of the mode TE and it is performed by
expressing the group velocity in polar coordinates.
[0110] A isotropic distribution of group velocities about a
predetermined crystal axis in an angular range of .pi./3 for a
triangular array, and of .pi./2 for a square array, has been
considered.
[0111] The parameter R has been varied between 0.15 and 0.45 by
steps of 0.3; as concern the parameter n, the values 1.5, 2, 2.5,
3.25 and 3.5 have been considered; the parameter .omega..sub.n has
been varied of steps of 1.multidot.10.sup.-4.
[0112] FIGS. 9a-9d show the group velocity distributions for R=0.21
and n varying in the set of values 2, 2.5, 3.25 and 3.5. The
abscissa axis represents the angles (in rad) and the ordinate axis
indicates the normalized value of the group velocity. For each
graph, the optimum value of o)n has been found by varying this
parameter by steps of 1.multidot.10.sup.-4. Electromagnetic
radiation of polarization TE and wavelength in the fundamental
photonic band has been considered.
[0113] Table I reports, for the different curves of FIGS. 9a-9d,
the value of n and the optimum value of .omega..sub.n, together
with the values of the square of variance (.sigma..sup.2), of the
kurtosis (K), of the height of the group velocity angular
distribution (.DELTA.h), and of the percentage of points (N)
originally in a range of .pi./3 about the considered propagation
direction that have been concentrated within an angular range of
.pi./9.
1TABLE I (R = 0.21) n .omega..sub.n .sigma..sup.2[rad.sup.2] K
[rad.sup.4] .DELTA.h N [%] 2 0.2954 2.5101 .multidot. 10.sup.-2
3.41 .multidot. 10.sup.-3 0.35 38 2.5 0.2377 2.0244 .multidot.
10.sup.-2 4.08 .multidot. 10.sup.-3 1.77 53 3.25 0.1839 1.7702
.multidot. 10.sup.-2 4.46 .multidot. 10.sup.-3 2.5 62 3.5 0.171
1.7793 .multidot. 10.sup.-2 4.8 .multidot. 10.sup.-3 2.56 64
[0114] It can be observed that the distribution of FIG. 9a is
unsuitable for photon guiding (according to the above-mentioned
conditions), while the distributions of FIGS. 9b-9d have values of
.sigma. and N suitable for photon guiding.
[0115] FIGS. 10a-10d show the group velocity distributions for
R=0.27 and n varying as above (2, 2.5, 3.25 and 3.5). As
previously, the value of .omega..sub.n providing the best
distribution has been chosen in each graph.
[0116] Table II reports, for the different curves of FIGS. 10a-10d,
the values of the characteristics parameters.
2TABLE II (R = 0.27) n .omega..sub.n .sigma..sup.2[rad.sup.2] K
[rad.sup.4] .DELTA.h N[%] 2 0.3059 1.6973 .multidot. 10.sup.-2 4.46
.multidot. 10.sup.-3 3.18 64 2.5 0.2479 1.3388 .multidot. 10.sup.-2
4.73 .multidot. 10.sup.-3 5.64 70 3.25 0.1931 1.1684 .multidot.
10.sup.-2 5.57 .multidot. 10.sup.-3 7.01 70 3.5 0.1798 1.2108
.multidot. 10.sup.-2 5.47 .multidot. 10.sup.-3 5.19 71
[0117] It can be observed that all the four distributions suitable
for photon guiding, that the distributions of FIG. 10b is a
preferable condition, and the distributions of FIG. 10c and FIG.
10d are more preferable conditions for photon guiding.
[0118] Table III reports the complete results of the simulation,
wherein R is varied between 0.15 and 0.45.
[0119] In particular, Table III shows, for each possible
combination of values of the parameters R, n, the value of the
.omega..sub.n that allows maximizing the guiding effect.
3TABLE III 1 2
[0120] In Table III, three areas have been delimited by different
lines, to identify preferred conditions of photon guiding, by
taking into account not only the values of .sigma. and N but also
the other parameters that characterize the group velocity
distribution. In particular, the thin line identify possible
conditions of photon guiding, double thin line identify more
preferred conditions of photon guiding and heavy line identify
still more preferred conditions of photon guiding.
[0121] Table IV indicates, for each couple of values (R, n) the
percentage of points of the group velocity distribution within an
angular range of .pi./9 about the considered propagation direction,
in relation with the optimum values of .omega..sub.n detected for
each couple, i.e. in relation to the values of Table III. The
analysis has been restricted to the ranges of values of more
interest. Areas corresponding to the preferred conditions of photon
guiding have been delimited by different kind of lines, as in Table
III.
4TABLE IV 3 4
[0122] Table V shows the value of variance .sigma..sup.2
corresponding to the sets of values of Table III. The analysis has
been restricted to the ranges of values of more interest. Areas
corresponding to different conditions of photon guiding have been
delimited by different kind of lines, as in Table III.
5TABLE V 5 6
[0123] This example shows that it is possible to determine terns of
values of R, n and .omega..sub.n that identify conditions of photon
guiding.
EXAMPLE 2
[0124] The same simulation has been performed for a photonic
crystal having a square array of holes. Differently from above, the
percentage of points originally in a range of .pi./2 about the
considered propagation direction that have been concentrated within
an angular range of .pi./6 is now considerd. The parameters R and n
have been varied between 0.15 and 0.45 and between 2 and 3.5,
respectively. Table VI reports the results of this simulation. In
particular, Table VI shows, for each possible combination of values
of the parameters R, n, the value of .omega..sub.n that allows
maximizing the guiding effect.
6TABLE VI 7 8
[0125] As above, areas corresponding to a possible condition and a
more preferred condition for photon guiding have been delimited by
a heavy line and a thin line, respectively.
EXAMPLE 3
[0126] X-crossing without cross-talk in a photonic crystal
according to the present invention has been demonstrated by a
numeric simulation.
[0127] The simulation has been performed by considering a
triangular array of holes filled with air, with a period a of 0.372
.mu.m. The bulk refractive index n is 3.5, the normalized spacing R
is 0.42 and the normalized frequency .omega..sub.n is 0.24. Two
beams of light A, B at wavelengths 1550 nm, having a width of about
5 .mu.m are propagated at 60.degree. from each other along two
crystal axes. Only the mode TM is considered. Propagation is
simulated in a plane x-y, with input of light at y=0 .mu.m. FIG. 11
shows the propagation of the two beams A, B within the photonic
crystal. It can be appreciated that the beams interfere with no
cross-talk.
EXAMPLE 4
[0128] A numeric simulation has been performed for demonstrating
the flexibility of the photon guiding technique of the present
invention, in particular the possibility of guiding light beams of
different dimensions in different positions of the crystal. The
same conditions of Example 3 are considered. Three beams of light
A, B, C of different width and at a wavelength of 1550 nm are
propagated along a same direction of the crystal. FIG. 12 shows the
collimated propagation of the three beams A, B, C.
EXAMPLE 5
[0129] A further numeric simulation has been performed for
determining the behavior of a light beam at the interface between a
photonic crystal according to the present invention and an
integrated waveguide of a known type. FIG. 13 shows, in a plane
x-y, a photonic crystal 800 according to the present invention and
a conventional integrated waveguide 801. The interface is
represented by a line 802 at coordinate y=0.
[0130] The photonic crystal 800 has a triangular array of holes
filled of air and a bulk refractive index n of 3.5. The normalized
spacing R is 0.33 and the operative wavelength is 1550 nm. The
normalized frequency .omega..sub.n is 0.188.
[0131] Waveguide 801 is a monomode waveguide, having a width of 4.5
.mu.m, a core refractive index of 3.5 and a refractive index step
of 0.01. This waveguide simulates, for example, a ridge waveguide.
The bulk material is the same as that of the photonic crystal.
[0132] A normalized intensity of the input beam is detected at a
detection point DP distant 5 .mu.m from the interface 802. It has
been observed that the beam intensity at DP is about 98.76% of the
input beam intensity. It has also been observed that losses at the
interface 802 are small.
* * * * *